Elemental Success by Jupiter Luna · AP Chemistry · Topic 2.2
Pick a cation and an anion and see them to scale, touching. The bar shows how hard the pair is to pull apart (q₁q₂/r, relative to Na⁺Cl⁻) and the graph shows the same thing as a potential-energy well. Compare pairs, rank four the way the AP exam asks, and test the model against real melting points.
| # | Compound | Ions | q₁ × q₂ | r₊ + r₋ (pm) | q₁q₂/r (× Na⁺Cl⁻) | m.p. (°C) |
|---|
Keep Na⁺. Record the pair with F⁻, Cl⁻, Br⁻ and I⁻. As the anion grows, what happens to r, to the bar and to the depth of the well? Do the melting points agree?
Na⁺F⁻ and Ca²⁺O²⁻ are almost the same size. Predict how many times deeper the Ca²⁺O²⁻ well is from the charges alone, then compare.
Rank KI, CaO, KCl and CaS by lattice energy. Group by charge first, then sort each group by size. Score it, then try new sets until you get three in a row.
Plot every compound's melting point against its bar. Which trends hold? Find the one compound that melts lower than q₁q₂/r predicts.
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